Unsaturated polyester resins, which are produced by hardening unsaturated polyesters obtained by polycondensation of unsaturated polybasic acid component and saturated polybasic acid component with polyhydric alcohol components (hereinafter, said unsaturated polyesters are referred to simply as unsaturated polyesters), with .alpha.,.beta.-ethylenically unsaturated monomers in the presence of a catalyst, have many advantageous characteristics, that is, relatively low costs of materials available for manufacture, ease of molding, excellent physical properties of hardened products, and no hazard of poisonous solvent because of the needlessness of solvent for application, and they have therefore various applications as molding materials. In particular, they are currently used widely as leading plastics available in combination with glass fiber for fiberglass reinforced plastics (FRP), which enjoy demands tending to increase year after year, as structural materials for boats at large, fishing boats, bathtubs, tanks, pipes, etc.
While having such various advantages, unsaturated polyester resins also have disadvantages, among which high shrinkage during hardening and poor alkali resistance have been pointed out. This shrinkage is not only a significant drawback for molding applications where high dimensional precision is required, but also a cause of cracks due to the increased internal strain, and further it presents the problem of deteriorating the resin adhesion to glass fiber. Secondly, unsaturated polyester resins are susceptible to acid- or base-promoted hydrolysis, particularly to erosion by alkaline solutions because high-molecular structure thereof is constructed through ester bonds. This has been a serious problem in application fields of unsaturated polyester resins. In addition, requirements for water resistance are getting more and more strict from the user side.
In order to prevent the shrinkage of unsaturated polyester resin during hardening, addition of a thermoplastic polymer thereto has so far been practiced, but this prior technique has many drawbacks and limited applications because the two polymers are incompatible with each other. As chemical-resistant unsaturated polyester resins, there have been known those which employ isophthalic acid as an acid component or employ bisphenol A or hydrogenated bisphenol A as a polyhydric alcohol component. Such polyester resins, though resistant to alkaline solutions, are liable to develop a milky turbidity because of the poor compatibility with the .alpha.,.beta.-ethylenically unsaturated monomer jointly used, and the molded products tend to crack because they are too hard.